As is well known, to improve the fuel efficiency or the driving performance of automobiles or to absorb the increase in weight accompanying safety measures or fuller options, reduction of the weight of automobiles is being sought. For this reason, for example, high strength steel sheets are being used to reduce the thickness of the body structure and thereby lighten the weight of the body.
For example, when using 590 MPa class high strength steel sheets to lighten the body, it is considered possible to secure the body strength while reducing the weight by about 40% compared with conventional steel plate. Very great results have been anticipated.
On the other hand, since an automobile receives force from bumps on the road surface while driving, impact when riding over road shoulders etc., and various other forces, torsional stiffness is required in addition to body strength. However, if using high strength steel sheets to reduce the thickness of the body structure, even if the body strength is secured, the torsional stiffness generally falls.
That is, in high strength steel sheets, the tensile strength of the steel plate is improved by the temperature history, ingredients, etc., but the Young's modulus of iron is constant and does not change. For this reason, if the body structure is reduced in thickness, the polar moment of inertia of area becomes smaller. As a result, the torsional stiffness falls. Therefore, when using high strength steel sheets etc. to maintain the body strength while reducing the thickness of the body so as to lighten the weight, it is also necessary to improve the torsional stiffness.
Regarding the torsional stiffness of the body, as art focusing on the front body structure, for example, art such as shown in PLTs 1 and 2 is disclosed.
Specifically, PLT 1 discloses providing hood ridge reinforcing parts which extend from front pillars forward to load input surfaces of strut towers and imaginary extensions which extend from front ends of the foot ridge reinforcing parts toward the front so as to pass through the input centers of the load input surfaces.
PLT 2 discloses to form strut housings as single parts and to join side members, hood ridges, a dash panel, and a cowl top panel to the strut housings to join them together.
Further, while not aimed at improvement of the torsional stiffness, art similar to that which is described in PLT 1 is disclosed in PLT 3. In PLT 3, the bottom ends of the front pillars are joined at the tops of the strut towers with the upper members. The front pillars and the hinge pillars and upper member which are positioned at their rears form ring shaped members with open center parts. Due to this, it is possible to effectively support the moment load which acts on the bottom ends of the front pillars.